Messier 74 (also known as NGC 628) is a picturesque example of a face-on spiral galaxy (type Sc) and is about the same size as our own Milky Way Galaxy. If we could view our home Galaxy from a similar perspective, we would find that the Milky Way has a somewhat more prominent central bulge and that its spiral arms would be wrapped a bit tighter than in M74.
Visible: DSS and Visible: Color - Gemini Observatory - GMOS Team
The black-and-white and color images of M74 clearly reveal the bright central bulge and the graceful spiral arms that symbolize these majestic galaxies. The spiral arms can be traced all the way into the luminous nucleus of the galaxy. The clumpy structure of the arms, particularly the one that winds towards the east and south (left and bottom), illustrates that the arms contain vast amounts of gas and (obscuring) dust and that these constituents provide the raw materials for massive star formation within individual molecular clouds. The bright point-like sources of light randomly scattered throughout the image are individual foreground stars within our Galaxy.
At first glance, the near-infrared image of M74 may appear rather "plain." However, this photograph reveals some important aspects of near-IR light and of spiral galaxies. Note how the spiral arms appear smooth and washed out when compared to the visible light photos. Hot and young stars dominate the arms of a spiral galaxy. However, near-infrared light traces the distribution of older and cooler stars within a galaxy. Hence, the prominence of the arms is reduced when studying spirals at near-IR wavelengths. Moreover, near-infrared light can pierce through the obscuration produced by the clumpy clouds of dust and gas within spiral arms. This also partly accounts for the smooth appearance of the arms in the near-IR. Once again, the point sources of light are foreground stars within our Milky Way Galaxy.
Mid-Infrared: IRAS and Far-Infrared: IRAS
The mid-IR and far-IR images of Messier 75 were taken by the path-breaking IRAS mission in 1983. Unlike the square pixels found in most of today's electronic solid-state detectors, those aboard IRAS were rectangular in shape. This fact accounts for the peculiar stretching (upper-left to lower-right) of individual peaks in the infrared emission. Moreover, the detectors were of relatively poor spatial resolution, as is easily discerned in the "pixelization" within the far-infrared image.
Nevertheless, one can make a few important observations. First, the central bulge of Messier 74 is easily seen as a peak of mid-infrared emission (above left). A secondary peak to the northwest (upper right) corresponds to one of the spiral arms (see visible-light photo for comparison). Massive star formation is taking place within this spiral arm, and the newborn stars are illuminating the surrounding dust clouds, which in turn re-radiate in the mid-infrared.
The far-infrared image (above right) is of poorer spatial resolution than the mid-IR picture, and we find that the infrared emission from the nuclear bulge and the northwestern spiral arm are blended into a large blob. This wonderfully illustrates the concept of spatial resolution. In general, if your detector size (pixel) is larger than the distance between two sources of light, the emission peaks will blend into one. For a telescope of a given aperture size, smaller pixels result in better spatial resolution, and allow you to distinguish better between a pair of nearby sources. Note that the slight luminosity enhancement to the south of the nucleus (in green) corresponds to the southern spiral arm seen in the DSS visible light image. Modest star formation is triggering the radiation of far-infrared light from the dust clouds surrounding the newborn stars.
The radio image reveals the spiral pattern of Messier 74, but finds that the nucleus of the galaxy is diminished when compared to the earlier photographs. This is evidence that M 74 is a normal galaxy, rather than an active galaxy. The latter galaxies typically have significant radio emission from the nucleus, due to synchrotron emission emanating from electrons in a strong magnetic field. Active galaxies are broadly classified as having active galactic nuclei (AGN), and include objects like quasars, blazars, and BL Lacertae objects. These are often thought to harbor black holes in their centers.
The brightest peak of radio emission, color-coded as white, appears to the southeast (lower left) of Messier 74 and appears to be unconnected with the spiral galaxy. A search of the NASA/IPAC Extragalactic Database (NED) reveals the existence of about ten discrete radio sources within the field of view. None of them, however, have been associated with an optical identification. The radio source is probably a background quasar, a speculation that remains to be confirmed by careful cross-identification with existing data and possibly by new spectroscopic measurements of optical targets to ascertain its distance.
Ultraviolet: ASTRO-1 UIT
Finally, the ultraviolet (UV) image of Messier 74 comes from a small Shuttle-borne telescope. UV light traces the population of young and medium-aged stars in a galaxy. The spiral pattern in M74 is spectacularly obvious in the UV image. Note that the spiral arms break into discrete clumps, revealing once again the patchy nature of star formation in large molecular clouds. Much of the UV light from newborn stars is effectively absorbed by the surrounding clouds of dust, and is re-radiated as infrared emission. Note the large clump of UV light about 5 arcminutes to the south of the galaxy nucleus. This area of widespread and massive star formation within the southern spiral arm is spatially coincident with similar emission peaks seen in the visible light DSS image and in the far-infrared photo examined earlier. The bright point source of UV light seen near the eastern (left) image of the photo is a foreground star within our Milky Way Galaxy. However, the bright patch immediately to the south (and slightly to the west) is another patch of star formation within M74. Both of these UV sources can also be seen in the visible light DSS image.